So I think this is roughly what antirez is talking about with 'registers' for a counter. From the article: The standard error of HyperLogLog is 1.04/sqrt(m),

So I think this is roughly what antirez is talking about with 'registers' for a counter. From the article: The standard error of HyperLogLog is 1.04/sqrt(m), where “m” is the number of registers used. Redis uses 16384 registers, so the standard error is 0.81%. My understanding is that the error is very small because you're averaging over 16,000 registers. Edit: also, I think you want the mean because it'll give you back a precise/fractional power of 2 that is pretty close to the actual count. The median would give you the nearest integer power of two, which is probably not a good estimate of the real count. Double edit: looks like there's some research around improving averaging by throwing away outliers: http://blog.notdot.net/2012/09/Dam-Cool-Algorithms-Cardinality-Estimation http://blog.notdot.net/2012/09/Dam-Cool-Algorithms-Cardinali...